This invention relates to high temperature furnaces for the growth of crystals having minimal structural imperfections. More particularly, the present invention is for an improved high temperature furnace for the growth of crystals where non-crystalline or polycrystalline material is first melted in an upright crucible having cylindrical symmetry. The crucible is maintained by the furnace in a thermal environment having radial symmetry perpendicular to the vertical axis of the crucible. Starting from the base of the crucible where a seed crystal is positioned, and continuing along the vertical axis, the temperature of the molten material is slowly lowered to induce crystal growth. During crystal growth, however, thermal gradients in the furnace are controlled such that at every plane perpendicular to the vertical axis of the crucible no thermal gradients are present. Therefore, the latent heat of fusion is extracted from the melt in the crucible in such a spatial and temporal relationship as to result in crystal growth from the base of the crucible to the top of the melt with a flat planar solid-liquid interface. This technique is referred to herein as the vertical solidification of melt (VSOM).
Several substantial industries are dependent on the production of large single crystals having high melting temperatures. To be useful these crystals must often be essentially free of structural defects. Such industries include those of: semiconductor electronics, where crystalline silcon, gallium arsenide and other materials are the beginning resource from which sophisticated electronic devices are manufactured; and, quantum electronics, where lasers are manufactured using for example, crystalline laser rods fabricated from Al.sub.2 O.sub.3 doped with chromium (ruby), or yttrium aluminum garnet doped with neodymium (Nd:YAG).
A widely used technique for growing such single crystals, i.e. boules, is the Czochralski method. This method involves the melting in a crucible of a purified meltstock, suitably doped, from which the single crystal is to be grown. Then a seed crystal having a predetermined orientation is dipped into the melt, and the heat input provided by the furnace to the melt is reduced while the seed crystal is rotated and slowly withdrawn from the melt. Unavoidable limitations imposed by the mechanics of withdrawing the boule from the melt are boule size limitations, faceted growth, and strained cores.
Due to the time, expense, and complexity involved in growing single crystals, increasing the size of produced boules becomes one of the more sought after features. For the larger the boule the more electronic devices or laser rods, for example, can be fabricated from a single boule. An unacceptable trade off, however, in attempting to achieve larger boule size is the introduction of imperfections in the boule's crystal structure. A technique which has been used to grow larger single crystals mainly from single component systems, e.g. sapphire, while at the same time trying to achieve high crystalline structural regularity is a so called heat exchange method (HEM). By this method, instead of withdrawing a seed crystal from the melt, a seed crystal is positioned in the melt at the base of the crucible above a helium cooled heat exchanger. The melt is kept at a constant temperature while growth of the crystal is controlled by the thermal gradient within the solid. In order to sustain growth, the thermal gradient within the solid has to be continuously increased. In a homogenous melt, heat propagation is in a straight path. Therefore, the solid-liquid interface at the beginning of the solidification takes a hemispherical shape and as growth progresses, the solid-liquid interface becomes more and more convex leading to a paraboloid-like shape in the last stages of the solidification process. The HEM has been found not to be suitable for the growth of large diameter laser quality single crystals of compositions which belong to binary systems, whose melting behavior is characterized by a steep liquidus.
To grow large diameter quality single crystals of compositions which belong to binary or even more complicated systems, the present invention utilizes a technique which can be described as the vertical solidification of melt (VSOM). In this technique, solidification commences at the bottom of a crucible containing the melt and continues in a vertical direction while maintaining planar solid-liquid interface until crystal growth is completed. A planar solid-liquid interface is essential for growing good quality single crystals but such an interface is difficult to maintain during the entire period of growth of a large diameter single crystal. VSOM requires the formation of a thermal field within the crucible whose isotherms must be parallel with the bottom of the crucible and have an upward gradient. The temperature within the liquid melt is slowly lowered allowing solidification about the seed crystal. The latent heat of crystallization is removed from the system by flowing through the crystallized solid into a heat sink. The heat flow through the heat sink must be equal to the net heat flow through the flat liquid-solid interface conducted through the already crystallized charge.